Modified ceramic enamel



Patented Sept. 29, 1953 MODIFIED CERAMIC ENAMEL Alden J. Deyrup, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application June 13, 1950, Serial No. 167,929

2 Claims.

This invention relates to a modifying material for a vitreous enamel and enamels containing the same. More particularly, it relates to a composition which when added to an enamel for light metals will greatly reduce any tendency of the enamel to tear and craw This is a continuation-in-part of my copending application Serial No. 87,343, filed April 13, 1949, now abandoned.

The tearing and crawling of enamels is a recognized problem in enameling with vitreous enamels, and consists in the breaking of the enamel film during firing, followed by contraction of the enamel sections between breaks. This may result in a fewloare spots or in large bare areas with heavy enameled edges around the bare areas. This phenomenon generally occurs during the early part of the firing operation as the enamel approaches and passes through the sintering temperature.

Tearing and crawling of vitreous enamels used in the enameling of sheet steel is greatly minimized by addition of alkali metal (sodium or potassium) nitrites to the enamel.

Vitreous enamels suitable for the enameling of sheet steel are not satisfactory for the enameling of light metals such as aluminum, aluminum alloys, or magnesium. Recently, vitreous enamels suitable for the enameling of light metals were discovered. These enamels are described and claimed in U. S. Patent No. 2,467,114, issued April 12, 1949, and comprise:

to 18 mole percent PbO 38 to 65 mole percent 8102 5 to 12 mole percent L120 0 to 22 mole percent NazO 0 to mole percent K20, and 0 to 11 mole percent TiOz (for purposes of the present invention, the T102 content may be as high as 20 mole percent), the total alkali metal oxide content (LizO-i-NazO-i-KzO) taining at least 80% of aluminum. As in the case of steel enamels, however, these enamels are subject to tearing and crawling, particularly when the enamel layer is comparatively thick. Moreover, sodium or potassium nitrites, which when added to steel enamels will minimize tearing and crawling, cannot be used with the new light-metal enamels since they cause profuse gas evolution as evidenced by frothing and blistering during the fusing of this enamel.

The use of the above-said light metal enamels has also shown up a hitherto unknown form of tearing, hereinafter to be referred to as microtearing. Micro-tearing appears first as numerous tiny cracks in the unfired enamel just below the sintering temperature. These tiny cracks divide the enamel into many tiny islands, usually from to 3 mm. in diameter. As the temperature increases, these islands separate further as their edges round off, and later start to flow together, but do not level out completely. Although micro-tearing is believed to be similar to ordinary tearing, instances have been found where reduction or elimination of ordinary tearing induced aggravated microtearing.

It is an object of this invention to provide a composition which may be added to the abovesaid light-metal vitreous enamels to reduce the tendency of said enamels to tear, crawl, or micro-tear.

It is another object of this invention to produce a light-metal vitreous enamel having a reduced tendency to tear, crawl, or "microtear.

It is still another object of this invention to provide a process for reducing the tendency to tear, crawl, or micro-tear of the abovedescribed light-metal vitreous enamels.

Other objects of the invention will appear hereinafter.

The objects of this invention may be accomplished by thoroughly mixing with the abovedescribed light-metal enamel, alkali metal silicate and alkali metal metaborate in such a ratio that the mixture contains 1% to 10%, by weight 01' the enamel, of an alkali metal silicate-alkali metal metaborate mixture consisting of 10% r030 e102 20 to 40% B203, and 30%.to NazO or K20 Any method of obtaining the above constituents in a soluble form in the enamel slip may be employed. The NazO and S102 may, for ex ample, be added as commercial water glass containing approximately 37% of a water-soluble sodium silicate having an empirical composition of about NazO-4Si0z and 63% water. However, any form of sodium silicate which will yield the desired concentration of soluble S102 is satisfactory. The metaborate, which has been found to be outstandingly superior for the purposes of this invention to other forms of boron may be added as such but is more conveniently formed in situ by adding boric acid or sodium tetraborate (borax) and sodium or potassium hydroxide. Sufficient alkali metal hydroxide is used to obtain the desired alkali metal oxide equivalent and assure the presence of metaborate.

If desired, the alkali metal metaborate and sodium silicate may be mixed with each other in the proportions in which they are to be contained in the enamel, and then together added to the enamel frit prior to the grinding thereof. In the event that other mill addition agents are to be added to the enamel frit, for example, ceramic pigments, dispersing agents, bonding agents, and the like, these substances may be premixed in suitable proportions with the alkali metal metaborate and sodium silicate.

The soluble alkali metal borates and silicates may be mixed with each other, with alkali metal hydroxide also, if necessary, in proportions to produce a mixture in which the silica, boric oxide, and alkali metal oxides are in the proportions of b0 S102 20% to B203 and 30% '00 N220 or K20.

If desired, materials yielding these proportions may be mixed with water in a convenient ratio, such as 20% to 70% of solids to 80% to 30% of Water.

Smaller amounts than the lower limits of the above ranges of SiOz, B203, and alkali metal oxide (based on the weight of the enamel frit) do not produce a sufiicient anti-tearing effect, and

Example I To 20 parts potassium hydroxide are added 22 parts boric acid (HsBOs), 43 parts of white lead (ZPbCO3'Pb(OH)2) and 15 parts of aqueous water-glass solution containing 63% water and 37% Na2O-4SiOz. The ingredients are thoroughly mixed and added in the proportion of one part of the mixture to 19 parts of the following enamel frit:

Percent by weight PbO 44.6 S102 29.6 L120 2.3 NazO 13.3 TlOz 10.2

and 0 to 1.9 parts of an inorganic pigment or opacifier.

The mass is thoroughly milled in a ball mill until a substantially homogeneous enamel 1s enamel frit and pigment or opacifier show material evidence of tearing and crawling, whereas the other test pieces enameled with the enamel containing the metaborate and silicate are free from tearing and crawling. In the above example, the white lead is added solely as a dispersing agent.

Example II To 35.4 parts of potassium hydroxide are added 39 parts boric acid and 25.6 parts of a water-glass solution containing 63% water and 37% Na2O-3.4SiO2. The several parts are thoroughly mixed and added in the proportion of one part of the mixture to 12.5 parts of the following enamel frit:

Percent by weight PbO 37.4 SiOz 30.0 LizO 2.3 NazO 13.8 K20 2.2 TiOz 12.7 Sb203 1.6

and 0.75 part of a colored inorganic glass pigment such as lead antimcnate.

The mass is thoroughly milled in a ball mill until homogeneous. Test pieces of aluminum or magnesium are sprayed with the above enamel mixture and similar test pieces sprayed with an enamel prepared solely from an aqueous slurry of the above-described enamel frit and pigment. The test pieces are dried and fired at a firing temperature between 920 F. and 980 F. The test pieces containing the enamel prepared solely from the enamel frit and pigment show considerable evidence of tearing and micro-tearing, whereas the other test pieces enameled with the enamel containing the metaborate and silicate are free from tearing and micro-tearing.

Example III To 4.7 parts sodium hydroxide are added 22.2 parts borax (Na2B4O'r10H2O), 47 parts of a water-glass solution containing 63% water and 37% Na2O-3ASiOQ, and 26.1 parts water. These materials are thoroughly mixed and added in the proportion of one part of the mixture to four parts of the following enamel frit:

Percent by weight PbO 34.2 S102 27.8 L 2.4 NazO 13.9 K20 2.8 T102 13.7 313203 2.8 BaO 2.4

and 0.4 part of a colored inorganic pigment or opacifier such as TiOz.

The mass is thoroughly milled in a ball mill until homogeneous. Test pieces of aluminum or magnesium are sprayed with the above enamel mixture and similar test pieces sprayed with an enamel prepared solely from an aqueous slurry of the above-described enamel frit and pigment or opacifier. The test pieces are dried and fired at a firing temperature between 920 F. and 980 The test pieces containing the enamel prepared solely from the enamel frit and pigment or opacifier show considerable evidence of tearing and micro-tearing, whereas the other test pieces enameled with the enamel containing the metaborate and silicate are free from such defects.

Reference in the specification and claims to parts, proportions and percentages, unless otherwise specified, refers to parts, proportions, and percentages by weight.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

I claim:

1. A vitreous enamel composition, suitable for the enameling of light metals comprising as an enamel frit:

10 to 18 mole percent PbO 38 to 65 mole percent SiOz to 12 mole percent L120 at least one of the group consisting of NazO and K20 in amounts as follows:

up to 22 mole percent NaaO up to 20 mole percent K20, and from traces up to 20 mole percent TiOz the total alkali metal oxide content (LizO-l-NmO-l-KzO) of the enamel composition being between 25 and 36 mole percent, and the ratio of the sum of said silica content and twice the said titanium oxide content of the composition to the said total alkali metal oxide content of the composition being between 1.8 and 3.0, all of the said oxide being melted into said enamel composition, and mixed therewith as agents to reduce tearing of said enamel during firing alkali metal silicate and alkali metal metaborate, taken from the class consisting of sodium and potassium in such a ratio that the mixture contains 1% to 10% by weight of the enamel frit of an alkali metal silicate-alkali metal metaborate mixture consisting of 10%-30% S102, 20%-40'% B203, and %-60% alkali metal oxide.

2. The method of preparing a vitreous enamel composition having a reduced tendency to tear which comprises mixing with an enamel frit com prising:

10 to 18' mole percent PbO 38 to mole percent SiOz 5 to 12 mole mole percent L at least one of the group consisting of NazO and K20 in amounts as follows:

up to 22 mole percent NaeO up to 20 mole percent K20, and from traces up to 20 mole percent TiO2 References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,467,114 Deyrup Apr. 12, 1949 FOREIGN PATENTS Number Country Date 564,597 Great Britain 1944 

1. A VITREOUS ENAMEL COMPOSITION, SUITABLE FOR THE ENAMELING OF LIGHT METALS COMPRISING AS AN ENAMEL FRIT: 